RICHARDS AND ROGERS. — ATOMIC WEIGHT OF ZINC. 175 



possible to obtain it ; accordingly, the temperature was allowed to 

 fall to about 200°, and the current of dry hydrobromic acid was 

 stopped. Soon air — dried by means of sulphuric acid, fused potash, 

 and phosphorus pentoxide — was substituted for the carbon dioxide, 

 the temperature being allowed to fall to about 150° to avoid any possi- 

 ble decomposition of the ziucic bromide; and this current of air was 

 continued for several hours until long after every trace of hydrobro- 

 mic and carbonic acid had been swept away. In order to " make 

 assurance doubly sure " the whole length of the tube, weighing-bottle 

 and all, was heated to 1 00° or more several times, in order to prevent 

 any possible occlusion of acid. When all was in readiness, the warm 

 boat was pushed by means of a long glass rod into the weighing bot- 

 tle, and the bottle was stoppered with the help of the same rod. The 

 apparatus was then pulled to pieces, the closed bottle was transferred 

 at once to a tight desiccator, and after a suitable rest of several hours, 

 it was weighed with all possible care. The various weighings are 

 tabulated below.* 



Weight of boat beforehand . 

 Weight of boat + bottle . . 

 The same + ZnBr2 after seven ) 



liours' cooling . . . . ) 

 After two hours more . . . 

 After two hours' standing in ) 



balance ) 



Weight of boat after experiment 

 Boat + bottle afterward . . 



Gain of boat 



Gain of boat + bottle . . . 



Average tare of boat + bottle ) 



+ ZnBr., <j 



Average tare of boat + bottle . 



Weight of ZnBrj in air . . . 



Correction to vacuum 20° and 



762 mm 



Weight of ZnBro in vacuum . 



Common Weights 

 Rigbt-hand pan, 



7693 

 ,9757 



2389 



2389 



,7693 

 9757 



Tare : 



Standard Weights, 



Left-hand Pan by 



Substitution. 



Grams. 



7.76937 



0.00372 



5.26754 



5.26754 



5.26757 



7.76938 

 0.00375 



Standard 

 Weights 



7.76932 

 0.00372 



5.26747 



5.26747 



5.26750 



7.76933 

 0.00375 

 0.00001 

 0.00003 



5.26748 

 0.00373 



5.26375 

 .00074 



5.26449 



Before having been treated with water, the bottle and boat were 

 allowed to stand for twenty hours in somewhat moist air. During 



* For detailed method see These Proceedings, XXVIII. 5. 



